The present invention relates to a system for controlling the speed of a motor. In particular, the invention relates to a system for digitally controlling the speed of single facer and double facer motors in a corrugator. The motor speed can be automatically brought up to a preselected minimum speed or selectively varied from any speed above the preselected minimum speed to a desired speed including an adjustable or a preset reference speed. The single facer motor speed can by synchronously controlled to follow the speed of the double facer motor.
In a corrugator for producing paperboard, liner is fed to a single facer machine. A corrugating medium is also fed to the single facer machine. In the single facer machine, the corrugating medium is adhesively bonded to the liner to form a single faced web.
The single faced web is fed by a conveyor to the upper level of a bridge. An identical single facer machine produces a second single faced web which is transported by a second conveyor to the lower level of the bridge.
Each single faced web exits from the bridge along a bridge guide. The bridge guide feeds both single faced webs through a preheater and glue machine to a double facer machine. The glue machine applies an adhesive bonding agent to the crests of the flutes on the single faced webs. The webs are bonded to each other with the lower web also being bonded to a third liner fed to the double facer machine. the double facer machine maintains the single faced webs in contact with each other with one web in contact with the third liner while subjecting these components to controlled heat to gelatinize a starch solution used as an adhesive bonding agent.
The lay-out of a corrugator combining the above components is described in detail in application Ser. No. 369,632 entitled "Corrugator" filed June 13, 1973 incorporated herein by reference, now U.S. Pat. No. 3,977,929.
Each single facer and double facer machine has a separate motor drive, motor tachometer and voltage comparator. A speed control system in the form of a motor operated potentiometer mechanically slaved to a reversible motor supervised by a control panel is associated with each single facer and double facer machine. Each motor operated potentiometer controls the speed of the associated machine motor by providing a set point speed signal indicative of the speed to which the machine motor is to be driven. The set point signal is fed to the voltage comparator. The tachometer senses the actual motor speed and feeds a signal indicative thereof to the voltage comparator. The voltage comparator compares the set point signal to the tachometer feedbach signal and generates an error signal for controlling the motor drive. The motor drive adjusts the motor speed to null out the error signal.
The single facer machine can be driven synchronously with the double facer machine as described in application Ser. No. 369,632. In synchronous operation, the double facer motor operated potentiometer is electrically coupled to the single facer voltage comparator. The single facer motor operated potentiometer is disconnected from the single facer voltage comparator. The set point signal from the double facer motor operated potentiometer is compared by the voltage comparator to the tachometer feedback signal from the single facer tachometer to produce an error signal for regulating the single facer motor speed in synchronism with the double facer motor speed.
The liner and the corrugating medium fed into the single facer machines are drawn from twin tension driven rolls mounted on separate mill roll stands. For example, there are two rolls of liner mounted on a mill roll stand for use in one of the single facer machines. When one roll of liner is being run into the single facer machine, the other roll is idle. When the running roll approaches depletion, the leading edge of the idle roll is spliced to the running liner entering the single facer machine. Liner from the idle roll is then drawn by tension to the single facer machine. The liner from the running roll is cut to complete the splice.
During the splice, the single facer motor speed should be reduced to reduce the tension on the idle roll. If the motor speed is not reduced, the tension can cause the splice to break. Typically, the speed of the liner entering a single facer machine is reduced to approximately 200 feet per minute. After the splice has been effected, the motor speed can be restored to the operating speed at the initiation of the splice. This should be done gradually to allow the idle roll to come up to speed without breaking the splice.
Heretofore, to reduce the motor speed to permit the splice to be effected and then restore the motor speed to the operating speed prior to splice, it was necessary to employ a separate preset potentiometer in addition to the motor operated potentiometer. The present potentiometer furnished the set point signal for reducing the motor speed during the splice. The speed control system was limited in that the motor operated potentiometer had to be disconnected while the motor was driven in response to the preset potentiometer. To restore the motor speed it was necessary to reconnect the motor operated potentiometer and disconnect the preset potentiometer.
The use of multiple potentiometers increases the system costs. In addition, changes in the set point signals on reducing and resuming speed are abrupt. An abrupt increase in speed could break a splice. Still further equipment must be provided in the motor drive to slow down the rate of change of the set point signals to prevent the splice from breaking.
Use of a motor operated potentiometer to vary the motor speed is limiting in several other respects. Elaborate relay controls are required to command the reversible potentiometer motor to mechanically adjust the potentiometer arm setting. The relay controls are costly and relatively unreliable. The use of motor operated potentiometers also limits the resolution of the system. In addition, the speed control system using motor operated potentiometers is not compatible with state of the art computer controls for a corrugator.